1. Field of the Invention
The present invention relates generally to the field of optical communication and more particularly to interleaver and de-interleaver devices used in dense wavelength division multiplexing (DWDM) applications.
2. Background Art
Optical communication has been an active area of development and is crucial to the enhancement of several key technological advancements, e.g., Internet and related new technologies. An important technology that enabled a higher data transmission rate is the dense wavelength division multiplexing (DWDM) technology. In many DWDM applications, there is a need to filter a group of signal channels such that these channels can be further separated, redirected to a different direction, or a fraction of these channels be dropped and added. In certain applications, interleavers and de-interleavers are used to bridge technologies with different channel spacing, enabling the usage of more economical solutions associated with larger channel spacing. In FIG. 1, an interleaver design (100) based upon a Gires-Toumois (GT) mirror (166, 168) and a Michelson interferometer is displayed. These interleavers separate a composite input optical signal (102) into two complementary signals in which the odd data channels are branched into one output (112) and the even channels are directed back into the input (102). In an interleaver application, the frequency space is divided into two groups of pass bands, one for output 1 and the other for output 2. Dingel and Izutsu described this prior art interleaver in a publication (Optics Letters, Jul. 15, 1998, vol 23, pages 1099-1101) and later in a U.S. Patent (U.S. Pat. No. 6,304,689 B1, Oct. 16, 2001). These documents are therefore incorporated herein by reference as relevant background materials. Another improved prior art interleaver is illustrated in FIG. 2. In this device, the input signal (202) is coupled to a 50% non-polarizing cubic beam splitter (220) trough a collimating lens such as a graded index lens (GRIN) lens (208). A GT mirror (266, 268) and a regular mirror (248) are used to form the interferometer. The odd channels return to one output fiber (212) through another lens (218) whereas the even channels return to the input fiber (202) through a lens (208). This type of interleavers and related devices has been disclosed in recent U.S. Patents (U.S. Pat No. 6,169,626 issued Jan. 2, 2001, and U.S. Pat. No. 6,275,322 issued Aug. 14, 2001). These patents are also incorporated herein by reference as relevant background materials. In FIG. 3, another prior art interleaver (300) based on a polarization beam splitter (PBS) and two GT mirrors is displayed. This prior art device has been disclosed recently in U.S. Pat. No. 6,169,604 issued on Jan. 2, 2001 to Cao and U.S. Pat. No. 6,310,690 B1 issued on Oct. 30, 2001 to Cao and Mao. These patents are therefore incorporated herein by reference as relevant background materials. In this prior art device, the input signal (302) is coupled to a PBS (320) through a collimating lens (308). The two arms of the device are two interferometers, one for each of the polarization components. For each interferometer, a polarization and phase-modified GT mirror (332-348) is used as two mirrors of the interferometer. The phases and Free Spectra Ranges (FSR) of the GT mirrors are modified/adjusted using waveplates 332 and 334. The relative phases of the two paths of each of the interferometers are adjusted by changing the orientations and thickness of the waveplates 332 and 334. Both interferometers are adjusted such that the odd channels return to one output fiber (304) through lens (308) whereas the even channels return to the other fiber (312) through another lens (318).
Another related prior art of a GT mirror has been disclosed in a pending U.S. patent application Ser. No. 09/796,565 filed on Mar. 2, 2001 by Qian. This patent is also incorporated herein by reference as relevant background material. As illustrated in FIG. 4, The GT mirror consists of optically contacted front and rear windows, with a precision, temperature insensitive spacer. The sealed cavity is filled with optical medium at certain density such that precise center frequency may be achieved Another cavity may be added in front of the GT such that the relative phase of the GT may be adjusted.
There are several areas of improvements of prior art devices 100 through 300. For instance, the use of a Michelson interferometer with one output returning to the same direction in 100 and 200 requires the use of an optical circulator in the optical xe2x80x9ccircuitxe2x80x9d in order to physically separate the output from the input. Another area of improvements is in the temperature stability of the devices. Device 100 is not based on a balanced design and will require temperature stabilization whereas devices 200 and 300 uses thin glass plates/wave plates for fine adjustments of the interferometers and these thin plates introduces reliability issues such as the use of epoxy and certain temperature related drifts. There is a need therefore for improved art such that more stable and reliable interleaver devices can be fabricated.
The present invention discloses a group of new optical designs of interleavers and de-interleavers. These new designs are based on Mach-Zehnder interferometers with one or two GT mirrors. The GT mirrors used in these designs have tunable/adjustable FSR and phases. Tuning and adjustment of FSR and phases of these improved devices are accomplished by changing the densities of the optical medium in air-spaced cavities. Another embodiment of the present invention utilizes multi-fiber pigtails and collimators in interleaver and de-interleaver devices. This embodiment enabled device integration such that two or more interleavers can be constructed using the same optical block. Another usage of interleaver and de-interleaver devices with multi-fiber pigtails/collimators is in the area of multi-channel add/drop applications. In this case, each interleaver functions as a pair of interleaver and de-interleaver.